The TPU$FILE_PARSE routine returns a string containing the fields
    requested of the file specified. The file is not required to
    exist when the parse is done. The intention of the TPU$FILE_
    PARSE routine is to construct a valid file specification from the
    information passed in through the file specification, the default
    file specification, and the related file specification.

    The routine uses the $PARSE system service to return the
    requested information.

    The TPU$FILE_PARSE routine is also called by DECTPU when the
    TPU built-in procedure FILE_PARSE is executed from TPU code. The
    return value of the built-in procedure is the string returned in
    the result-string argument.